Disk drives are a cost effective data storage system for a computer or other data processing device. As shown in FIG. 1, a disk drive 10 comprises a magnetic recording medium, in the form of a disk or platter 12 having a hub 13 and a magnetic read/write transducer 14, commonly referred to as a read/write head. The read/write head 14 is attached to, or formed integrally with, a suspension arm 15 suspended over the disk 12 and affixed to a rotary actuator arm 16. A structural arm 18, fixed to a platform 20, is pivotably connected to the actuator arm 16 at a pivot joint 22. A voice coil motor 24 drives the actuator arm 16 to position the head 14 over a selected location on the disk 12.
As the disk 12 is rotated at an operating speed by a spindle motor (not shown) the moving air generated by the rotating disk, in conjunction with the physical structure of the suspension arm 15, lifts the read/write head 14 away from the platter 12, allowing the head to glide or fly on a cushion of air slightly above a surface of the disk 12. The flying height of the read/write head over the disk surface is typically less than one micron.
An arm electronics module 30 may include circuits that switch the head function between read and write operations and write drivers that supply write current to the head 14 during the write operation, for effecting a change to magnetic domains of the disk 12 to store data thereon. The arm electronics module 30 may also include a preamplifier electrically connected to the head 14 by flexible conductive leads 32. During read operations the preamplifier increases the read signal signal-to-noise ratio by amplifying the read signals produced by the head 14. In the write mode, the preamplifier scales up the relatively low voltage levels representing the data bits to be written to the disk to a voltage range of about +/−6 to +/−10V. The preamplifier also shapes the voltage levels to optimize the data writing process. The components comprising the electronics module 30 may vary according to the disk drive design, as understood by persons familiar with such technology.
To minimize signal losses and noise induced into read signals produced by the head 14 during read operations, the electronics module 30 is advantageously located proximate the head 14. A side surface of the structural arm 18 is a preferred location for mounting the electronics module 30, as shown in FIG. 1.
FIG. 2 illustrates a magnetic transducer or head 14, typically comprising a write head 14A for producing magnetic transitions in the disk 12 and a read head 14B for reading the magnetic transitions in the disk 12. During a write operation, current through the write head 14A alters magnetic domains of ferromagnetic material in a thin film 52 for storing the data bits as magnetic transitions. Data bits are stored on the platter 12 in sectors 40 on concentric tracks 42. See FIG. 1. Typically a sector contains a fixed number of bytes (for example, 256 or 512). A plurality of sectors are commonly grouped into a cluster. During read operations the read head 14B senses the magnetic transitions to determine the data bits represented by the magnetic transitions.
In other data storage systems the head 14 operates with other types of storage media (not shown in the Figures) comprising, for example, a rigid magnetic disk, a flexible magnetic disk, magnetic tape and a magneto-optical disk.
The disk drive read head 14B comprises either a magneto-resistive (MR) sensor or an inductive sensor. The former produces a higher magnitude output signal in response to the magnetic transitions, and thus the output signal exhibits a greater signal-to-noise ratio than an output signal produced by the inductive sensor. The MR sensor is thus preferred, especially when a higher areal data storage density is desired.
During read operations the read head 14B is biased by a DC (direct current) voltage of about 0.04V to 0.2V supplied by the preamplifier to read head terminals 54A and 54B via the conductive leads 32. Magnetic domains in the thin film 52 passing under the read head 14B alter a resistance of the magneto-resistive material, imposing an AC (alternating current) component on the DC bias voltage, wherein the AC component represents the read data bits. The AC component is detected in the preamplifier, but has a relatively small magnitude (e.g., several millivolts) with respect to the DC bias voltage.
According to another embodiment, the preamplifier supplies a constant current bias to the read head 14B, in lieu of the constant voltage bias described above. The bias current develops a constant voltage across the resistance of the magneto-resistive material, where the developed voltage is dependent on the value of the head resistance.
As described, the preamplifier provides not only read head signal amplification, but also supplies the fixed bias voltage (or current) for the read head 14B. As known in the art, there exist other applications in which a preamplifier amplifies a sensor signal and also supplies a sensor bias.
Drive manufacturers and system level users have an interest in measuring a read head resistance (RMR), i.e., a resistance of the MR sensor. If the RMR value exceeds a critical value RMR MAX a gross failure of the head is suspected. Detection of an excessively large head resistance is commonly referred to in the industry as “open head detection.” Generally, the head resistance ranges from about 5Ω-500Ω. A resistance greater than about 1 kΩ is considered problematic. An optimum head bias is also related to the head resistance, and thus knowing the head resistance permits the disk drive manufacturer to employ the optimum bias voltage.
In certain applications one or more diodes are connected across the signal terminals 54A and 54B to protect the head 14B during electrostatic discharge (ESD) events. An ESD voltage shorts the diodes thereby avoiding ESD current flow into the head 14B.
In an application where the preamplifier biases the read head 14B with a constant current IMR, current through the terminals 54A and 54B develops a voltage VMR=IMR×RMR across the head 14B. Since the voltage VMR developed by a properly functioning head is less than the diode turn-on voltage, the ESD-protection diodes remain in an off condition when the head resistance is within an expected range. A head resistance greater than a nominal value causes the constant current IMR to develop a voltage across the diodes that exceeds the diode turn-on voltage. A voltage mode comparator detects a head potential VMR in excess of a voltage threshold by determining a state of the ESD diodes. If the ESD diodes are in an on state the head resistance exceeds the nominal value.
In an embodiment in which the preamplifier delivers a constant voltage bias VMR to the head 14B, the voltage mode comparator technique cannot be used for open head detection since the read head voltage is fixed. Instead, a current mode comparator determines the current drawn by the head, in response to the head resistance RMR and the constant voltage bias VMR, to determine the head resistance. Since the current and the voltage are known the head resistance is calculated from the equation.
                              I          MR                =                              V            MR                                R            MR                                              (        1        )            
Given the inverse relationship between the head current and the head resistance (as indicated in equation (1)), a current IMR MIN detected by the current mode comparator indicates a head resistance RMR MAX (VMR being a known quantity). If the current mode comparator measures a head current less than IMR MIN then the head resistance exceeds RMR MAX. It is known that such a head resistance measurement technique may be undesirable not only because it introduces a dependency on the predetermined bias voltage VMR, but also because it requires the use of an undesirably small reference current IMR MIN that may compromise accuracy of the measured head resistance.